A better way to install automation in classified areas

Instead of employing protection methods, it’s often better to move automation systems to less hazardous areas and to use components rated for use in these locations.

Steve Massie

09/08/2015

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Classified areas are often found in industrial plants due to the presence of hazardous gases and dust. When automation components are installed in these areas, certain rules, regulations, and design standards must be followed to ensure safety.

Hazardous areas are classified according to either National Electrical Code (NEC) or the International Electrotechnical Commission (IEC). Table 1 shows a simplified version of this classification system, and much greater detail can be found within the detailed regulations, specifically NEC Article 500 and IEC 60079.

The NEC standards are useful for reference. As shown in Table 1, Division 1 is more hazardous than Division 2 because hazards are more likely to be present. Therefore, automation components installed in Division 1 require a higher level of protection than those installed in Division 2. Three main methods are used to implement levels of protection as required in classified areas: explosion proof, purging, and intrinsic safety. All these methods of protection require careful design-and often expensive components, installation, and maintenance.

It's useful to look at these three protection methods before examining an alternative way for installing automation components in classified areas.

Explosion-proof approaches

Explosion-proof enclosures and conduit systems protect automation components in hazardous areas through two methods. First, they are sealed to limit the amount of gas or dust that can enter the enclosure or conduit system. Second, they must have the structural integrity to contain an explosion so it doesn't propagate throughout the classified area. These enclosures are typically used in Division 1 areas.

Table 1: NEC Divisions and IEC Zones

Frequency of Hazard Occurrence

NEC Classification

IEC Classification

Continuous hazard

Division 1

Zone 0

Intermittent or periodic hazard

Division 1

Zone 1

Abnormal condition hazard

Division 2

Zone 2

Table courtesy: Hoffman

This method of protection has been in use for many decades and is thus well understood by many engineers, designers, and plant maintenance personnel. Many suppliers provide explosion-proof enclosures and conduit systems, and the design standards for such systems are readily available.

But compared to the standard National Electrical Manufacturers Association (NEMA) 4 enclosures, explosion-proof enclosures are very expensive, quite large, and very heavy (see Figure 1). Because these enclosures are sealed, the only practical method to dissipate heat is through the enclosure itself, limiting the type of components the enclosure can house, or requiring the enclosure to be oversized which further adds to costs.

Any operator interface components must be rated for use in the area, which precludes the use of any type of graphical interface in a Division 1 area. Graphical interfaces can be mounted inside the enclosure and viewed through a window, but this limits visibility.

Great care must be taken during installation of the enclosures and conduit systems to make sure proper sealing is preserved, and conduits must often be potted at transitions from Division 1 to Division 2 areas. Hot-work permits and accompanying precautionary measures are required to open explosion-proof enclosures, making servicing of the components inside the enclosure very expensive and difficult.

Finally, regular inspections of explosion-proof enclosures and conduit systems are required in some classified areas and strongly recommended in others. These inspections are necessary to verify the integrity of the sealing systems and routinely reveal the need for maintenance.

Purging problems

Another method for installing automation components in hazardous areas is to purge enclosures with compressed air or an inert gas. Purge systems don't allow hazardous gases or dusts to enter an enclosure because the interior is always under positive pressure.

Depending on the type of purge system, this method allows standard enclosures and unrated automation components to be used in Division 1 and Division 2 areas. But purge systems can be quite expensive, particularly when they are used in Division 1 areas (see Figure 2).

The cost doesn't stop with installation, as purge systems require a constant supply of compressed air or an inert gas. This approach makes maintenance of the components within the enclosure problematic because the enclosure cannot be opened during normal operation.

Limitations of IS options

Intrinsically safe (IS) systems can be safely installed in Division 1 and 2 hazardous areas because the components and wiring systems cannot release sufficient energy to ignite gas or dust in the area. Unlike explosion-proof or purge systems, this method of protection permits many types of maintenance during normal operation.

Because each component must be rated for use in the area, the range of items available for use is limited. Many common automation components, such as programmable logic controllers (PLCs) and motor drives, aren't available with an IS rating. And even when IS-rated components are available, there is often an added charge from the supplier.

The design of IS systems requires a high level of engineering expertise, adding to implementation time and expense. Transitions from one classified area to the next often require IS barriers, further adding to the complexity of these systems.

Avoid the hazard

As related in the above sections, all three of the leading protection methods require detailed design and often considerable expense for system purchase and installation. And the expense doesn't stop with installation, because IEC 60079 requires not only initial inspections of components installed in classified areas, but also ongoing periodic inspections, typically done annually. The inspections often reveal areas requiring maintenance, further adding to expense.

Realizing this, many suppliers began to introduce components within the past few years which can be safely installed and operated in Class 1, Division 2 (or Class I, Zone 1) classified areas. There is a significant difference between those two divisions even though they are in the same class, and the requirements for applicable devices and components are also significant.

Learn more about automation components rated for Division 2 use as well as information about component selection and application.